1. Field of the Invention
The present invention relates to a planar-type avalanche photodiode (referred to as APD hereinafter) that has high sensitivity and a wide dynamic range as well as high reliability suitable for use in high-speed optical fiber communication.
2. Description of the Prior Art
As a high-speed high-sensitivity optical receiver for use in the next generation optical fiber communication system, a planar-type superlattice APD shown in FIG. 6 has been reported (IEEE Photonics Technol. Lett., Vol.8, pp.827-829, 1996; Japanese Patent Application No.299996/1994/Japanese Patent Application Laid-open No. 312442/1995).
In such a device that is a first example of the prior art herein, a higher gain-bandwidth product (GB product) and lower noise are brought about through the enhancement effect of an InAlAs/InAlGaAs superlattice multiplication layer on the ionization rate ratio. At the same time, higher reliability is also produced therein by a structure in which only a stable InP planar pn-junction is exposed to the surface.
In such a conventional planar-type superlattice APD, a Ti-implanted guard-ring is formed in order to suppress edge-multiplication. The operation principle thereof is as follows. Due to Ti implantation, the p-concentration in a p-InP field buffer layer of the guard-ring section decreases locally. Taking advantage of this decrease, the punch-through voltage of a depletion layer to a photo-absorption layer in the guard-ring section is reduced, as shown in FIG. 5, and thereby the bias voltage applied to an edge section is spread over the photo-absorption layer section, suppressing the edge electric field increase.
If the spreading of the voltage over the photo-absorption layer hereat is excessive, however, the field strength of the photo-absorption layer in the edge section becomes too high and passes over the tunneling dark current limit (normally 200 kV/cm) and the dark current of the APD device is increased. This indicates that there exists the optimum value for the dose of Ti implantation. Under such conditions, in order to provide a wider dynamic range for the high frequency response of the APD, in other words, to obtain a high frequency response (a 3 dB bandwidth above 10 GHz) even at a low bias and a low multiplication factor M (M&gt;.about.1.5), the maximum field strength in the photo-absorption layer at the time of breakdown should be set as high as .about.150 kV/cm or so. In this case, however, when the bias voltage is spread over the photo-absorption layer in order to bring about the above-mentioned guard-ring effect, there arises another problem that the tunnel dark current is liable to generate in the photo-absorption layer section of the guard-ring because the first setting of the field strength itself is already relatively high.
Accordingly, the conventional planar-type superlattice APD has a problem that a range of the optimum value for the dose of Ti implantation mentioned above tends to become small, depending on the setting of the maximum field strength in the photo-absorption layer.